Potential of <i>Eugenia brejoensis</i> (Mazine) Essential Oil in Combating Multidrug‐Resistant (MDR) <i>Acinetobacter baumannii</i> and <i>Pseudomonas aeruginosa</i>

The genus Eugenia has a long history of use in traditional medicine for treating various conditions, including infectious diseases, gastrointestinal disorders, and skin problems. Essential oils derived from Eugenia species are known for their medicinal properties and have been studied for their antimicrobial and bioactive potential. This study aimed to evaluate, through in vitro tests, the antibacterial, antibiofilm, and antibiotic-modulating effects of Eugenia brejoensis essential oil (EOEb) against multidrug-resistant (MDR) clinical isolates of Acinetobacter baumannii and Pseudomonas aeruginosa from COVID-19 patients. In addition, we sought to analyze its toxicity and survival rates through in vivo tests in an invertebrate model using Tenebrio molitor. The EOEb was extracted via hydrodistillation and analyzed by gas chromatography-mass spectrometry (GC-MS) and flame ionization detector (FID). Clinical isolates of A. baumannii and P. aeruginosa were identified using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), and their resistance profiles were determined using the Vitek 2 system. The in vitro tests were conducted using the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined using microdilution and plating methods. Biofilm formation and inhibition assays were performed using crystal violet staining. Synergistic effects of EOEb with ciprofloxacin and gentamicin were evaluated using the checkerboard assay. The antibacterial effect of EOEb was tested in vivo using the T. molitor (mealworm) larvae model to assess toxicity and survival rates. The major constituents of EOEb were rosifoliol (16.47%), guaiol (12.69%), and (E)-caryophyllene (11.97%). All bacteria exhibited an MDR profile. EOEb showed significant antibacterial activity against MDR strains, with MIC and MBC values ranging from 0.512 to 4.096 mg/mL. It also effectively inhibited biofilm formation at concentrations between 0.512 and 4.096 mg/mL. EOEb exhibited synergistic effects with ciprofloxacin against A. baumannii and with gentamicin against P. aeruginosa, as indicated by fractional inhibitory concentration (FIC) indices close to 0.5. The EOEb demonstrated low toxicity in the T. molitor model, with a survival rate of approximately 70%. The EOEb exhibits notable antimicrobial and biofilm-inhibiting properties against MDR pathogens. Its low toxicity and synergistic effects with conventional antibiotics suggest its potential as a therapeutic alternative for combating antibiotic-resistant infections.